I am working on orthorectifying old CORONA satellite images. The original data is a high resolution, and I was able to create a good orthorectification using the digital line camera mode in building the RPCs. However, orthorectifying takes a very long time with this high-res image and I'd like to be able to fiddle more easily. I resized the image (xfac and yfac =0.3) and created new GCPs for this image. Now when I ortho. this lower-res image it is consistenly far off (i.e., many many kms off) from where it should be. I've tried changing the focal length and pixel size, but no luck. Any ideas? Thanks.